1. Field of the Invention
The present invention relates to a magnetic thin film memory, a method of writing information in this magnetic thin film memory, and a method of reading the information written in this magnetic thin film memory.
2. Related Background Art
Recently, the development of devices including a thin film magnetic head to which the magneto-resistance effect is applied has been promoted. Among them, a magnetic thin film memory using a magnetoresistive thin film is noted, which can be substituted for a DRAM or an EEPROM used at present. Since the magnitude of resistance of a magnetoresistive thin film can largely be changed by the state of magnetization of the magnetism, a nonvolatile solid memory can be realized by combining the magnetoresistive thin film with a semiconductor device such as a transistor.
As a conventional magnetic thin film memory, for example, a memory is proposed in Japanese Patent Application Laid-Open No. 6-84347, which is formed by connecting a magnetoresistive thin film to the source electrode of a field effect transistor (hereafter, referred to as "FET"). The configuration of this memory is shown in FIG. 1. In FIG. 1, the numeral 1 denotes a magnetoresistive thin film, and in order to show the positions in the circuit, the magnetoresistive thin films are denoted by the subnames like 1aa, 1ab, 1ac, 1ba, 1bb and 1bc, but hereafter, in the case when the positions in the circuit are not especially specified, it is simply referred to as the magnetoresistive thin film 1. Furthermore, the other numerals are also referred to similarly. The numeral 2 denotes an FET; 3, a first bit wire; 5, a second bit wire; 4, a word wire; and 6, a resistor. The word wire 4 is provided in the lateral direction of FIG. 1 and is connected to the gate electrode of the FET 2, and the first bit wire is provided in the longitudinal direction of the figure and is connected to the drain electrode of the FET 2. Furthermore, the source electrode of the FET 2 is connected to the second bit wire 5, and further, the magnetoresistive thin film 1 is connected to the second bit wire 5. The other end of the magnetoresistive thin film 1 which is not connected to the second bit wire 5 is connected to a grounding source.
FIG. 2A and FIG. 2B show cross sectional views taken along the line A--A' of the magnetoresistive thin film 1ac shown in FIG. 1. In the figures, arrows show the direction of the magnetic field, and the mark ".smallcircle." in the circle (i.e., the mark ".circleincircle.") shows the state where the current flows from the back to the front of the figure, and the mark "x" in the circle (i.e., the mark "x") shows the state where the current flows from the front to the back of the figure. Furthermore, FIG. 3A and FIG. 3B show the configuration of the magnetoresistive thin film 1.
The magnetoresistive thin film 1, as shown in FIG. 3A and FIG. 3B, comprises a giant magnetoresistive thin film in which a magnetic layer a with a large coercive force and a magnetic layer b with a small coercive force are stacked several times with interposition of a nonmagnetic layer c therebetween. The resistance value of the magnetoresistive thin film 1 is characterized in that it is small when the direction of magnetization of the magnetic layer a and the direction of magnetization of the magnetic layer b are the same, and that it is large when the direction of magnetization of the magnetic layer a and the direction of magnetization of the magnetic layer b are opposite.
In FIG. 1, when the information of "1" is written in the magnetoresistive thin film 1ac, a potential of +V.sub.3 is applied to the first bit wire 3c. In this case, when a voltage of V.sub.4 is applied to the word wire 4a, the FET 2ac is turned on, and a comparatively large current I.sub.1 flows into the magnetoresistive thin film 1ac and the second bit wire 5ac. By this current I.sub.1, a magnetic field H.sub.1 is applied to the magnetoresistive thin film 1ac, and the magnetic layer b with a small coercive force which is shown in FIG. 3A and which relates to the writing of the magnetoresistive thin film 1 is magnetized to the left which is the direction of the magnetic field H.sub.1.
On the other hand, when the information of "0" is written in the magnetoresistive thin film 1ac, a potential of -V.sub.3 is applied to the first bit wire 3c. In this case, when a voltage of V.sub.4 is applied to the word wire 4a, the FET 2ac is turned on, and a comparatively large current I.sub.0 flows into the magnetoresistive thin film 1ac and the second bit wire 5ac in the opposite direction (from the front to the back in the figure) of the above I.sub.1. By this current I.sub.0, a magnetic field Ho is applied to the magnetoresistive thin film 1ac, and the magnetic layer b with a small coercive force which relates to the writing of the magnetoresistive thin film 1 is magnetized to the right which is the direction of the magnetic field H.sub.0 as shown in FIG. 3B.
Since the FET 2ac is set to be turned on only when a proper voltage is applied to the word wire 4a, the current does not flow into other magnetoresistive thin films 1 connected to the first bit wire 3c.
Furthermore, since the current does not flow through the first bit wires 3 other than the first bit wire 3c, the current also does not flow into other magnetoresistive thin films 1 connected to the word wire 4a. Since the magnetic layer a with a large coercive force is initialized so that the direction of magnetization may face to the right at all times, the magnetoresistive thin film 1 is set to have a large resistance when the information of "1" is written in it and to have a small resistance when the information of "0" is written in it.
On the other hand, when reading the information written in the magnetoresistive thin film 1ac, a current I.sub.3 is supplied into the first bit wire 3c, and a voltage V is applied to the word wire 4a so as to turn on the FET 2ac. Consequently, since the current I.sub.3 flows only into the magnetoresistive thin film 1ac from the top to the bottom in the FIG. 1, the voltage V.alpha..beta. between the positions .alpha. and .beta. at this moment is measured. In the cases of the same and opposite directions of magnetization of the magnetic layer a and the magnetic layer b constituting the magnetoresistive thin film 1ac, the resistance values of the magnetoresistive thin film 1ac are different, and therefore, the values of the voltage V.alpha..beta. are also different. Accordingly, it is possible to judge whether the information read from the magnetoresistive thin film 1ac is "0" or "1" by the magnitude of the voltage value of the voltage V.alpha..beta..
FIG. 4 is a circuit diagram of the configuration of the conventional magnetic thin film memory shown in FIG. 1. The reference characters M101 to M104 denote MOS (metal/oxide/semiconductor) FET's; R101 to R104, magnetoresistive thin films; W101, a word wire; B101 to B104, bit wires; and G, a grounding wire.
FIG. 5 shows a circuit diagram in which circuits shown in FIG. 4 are arranged in the form of a matrix. One end of each of the magnetoresistive thin films R101 to R109 is connected to either the source electrode or the drain electrode of each of the MOSEFT M101 to MOSFET M109, and the other end of each of the magnetoresistive thin films R101 to R109 is connected to the grounding source. The drain electrodes of the MOSFET M101 to MOSFET M109 are connected to the bit wires B101 to B103, and the word wires W101 to W103 are arranged to the gate electrodes of the MOSFET M101 to MOSFET M109. Furthermore, the reference characters J101 to J103 denote writing wires. Accordingly, in the circuit in FIG. 5, when the MOSFET M101 and the magnetoresistive thin film R101 are magnetic thin film memory elements, wires necessary for constituting the magnetic thin film memory elements are 4 lines of the word wire W101, the bit wire B101, the grounding wire G, and the writing wire J101.
Furthermore, FIG. 6 shows one example of the device structure of the circuit shown in FIG. 4. In the device structure, the above magnetic thin film memory element is referred to as "memory cell". The numeral 101 denotes a magnetoresistive thin film; 102, a writing wire; 103, a gate electrode; 104, a bit wire; 105, a connecting electrode between the bit wire 104 and an n-type region 109; 106, a connecting electrode between the magnetoresistive thin film 101 and the n-type region 109; 107, a grounding wire; and 108, a field oxide film region composed of SiO.sub.2 for electrically separating adjacent memory cells on a p-type Si substrate 110.
As mentioned above, an conventional magnetic thin film memory uses a wire functioning as a bit wire and as a writing wire, and the current passed through the bit wire when writing the information may also flow into the magnetoresistive thin film itself since the magnetoresistive thin film is made of a metal material, whereby the magnetic field necessary for the writing cannot effectively be applied to the magnetoresistive thin film. Therefore, it is required that a writing wire is newly provided near the magnetoresistive thin film in addition to the bit wire, and that the current is supplied into this writing wire to perform the writing by using the generated electric field. Furthermore, each electrode of a transistor constituting one memory cell is independent from each electrode of a transistor of an adjacent memory cell. Accordingly, in order to constitute a magnetic thin film memory element, 4 lines of wires of a bit wire, a word wire, a writing wire and a grounding wire are necessary.
In the device structure of the conventional magnetic thin film memory shown in FIG. 6, the electrodes connected to the n-type region formed on a p-type semiconductor substrate for one memory cell are two electrodes of the connecting electrode for connecting the bit wire and the n-type region and the connecting electrode for connecting the magnetoresistive thin film and the n-type region. Normally, in order to connect these two electrodes to the n-type region, contact holes are used. Accordingly, in the device structure of the above conventional magnetic thin film memory, two contact holes are needed for one memory cell. Considering the misregistration of the mask of a light exposure apparatus used when forming a contact hole, it is necessary that the area for forming a contact hole is wider than the actual occupied area of a contact hole. Accordingly, when the number of contact holes is increased, the occupied area of the memory cell becomes larger.
Furthermore, in the conventional magnetic thin film memory, a field oxide film region is provided for separating the adjacent memory cells. It is necessary that this field oxide film region has a sufficiently wide area for surely separating the adjacent memory cells on the semiconductor substrate. The contact hole and the field oxide film region need areas most when forming a memory cell, and the increase of the number of contact holes and the formation of the field oxide film region increase the occupied area of the memory cell itself as a result. Thus, a conventional magnetic thin film memory has a complex structure since the number of lines of wires is large, and it is difficult to improve the degree of integration, since the area of a memory cell is wide.